14 
The Coeorado Experiment Station 
sections of the state. There is a pronounced variation in the length 
of this period. Grand Junction has the longest season (average 
183 days) of any locality in the state, while on the highest peaks 
there is no day in the year without frost. Those sections of Colo¬ 
rado with a frostless season averaging above 150 days are the 
lower Grand Valley, a limited area, on the North Fork of the Gun¬ 
nison about Paonia, the lower McElmo Valley, the Arkansas Val¬ 
ley up as far as Canon City, and an area along the foothills from 
below Denver to north of Boulder. The tenderer crops are, of 
course, grown with best success in those sections in which the 
frostless season averages 150 days or more. However, this state¬ 
ment must not be taken to mean that all portions of the state with 
a frostless season of 150 days and over are climatically well 
adapted to tender crops like peaches and apricots, nor that some 
localities with a slightly shorter average frostless season can not 
grow such crops with considerable success. There are other cli¬ 
matic conditions besides length of growing season to be consid¬ 
ered, for example, coldness and dryness of the winter season, the 
character of the springs, and total effective heat during the period 
free from frost. Those stations with a frostless season of 75 days 
or less are usually such as may experience frost any month of the 
year. However, over the lower part of this extensive area, which 
has a growing season averaging less than 75 days, alfalfa, timothy, 
potatoes, Canada field peas, the small cereals, gooseberries, cur¬ 
rants, raspberries, strawberries, and a large list of vegetables are 
grown with profit. 
It is obvious that two stations with the same number of frost¬ 
less days may have temperature values during this period that dif¬ 
fer widely in their effectiveness in determining the rate of plant 
growth. Daily mean temperatures may be the same, but the maxi¬ 
mum and minimum temperatures different. Furthermore, although 
the number of hours of a day during which the temperature is suf¬ 
ficient to promote plant growth may be the same in two localities, 
the total effective heat in the two instances may differ widely. 
Thus it is realized that it is difficult to relate ordinary tempera¬ 
ture data, in their usual recorded form, to plant growth; but a 
number of attempts* have been made to do this in a manner which 
is more satisfactory than the rough, qualitative method here em¬ 
ployed. 
* Livingston, B. E., and Livingston, Grace J. Temperature coefficients in plaDt geog¬ 
raphy and climatology. Bot. Gaz. 56 :349-375. 1913. 
MacDougal, D. T. The auxothermal integration of climatic complexes. Amer. Jour. 
Bot. 1 :186-193. 1914. 
Livingston, B. E. Physiological temperature indices for the study of plant growth 
of relation to climatic factors. Physiol. Res. 1 :399-420. 1916. 
